A conventional mobile device such as a notebook computer or a cellular phone, which is operated by a storage battery, becomes inoperative due to insufficient remaining battery charge (battery rundown). To avoid this situation, it is proposed to detect a remaining battery charge of a mobile device by comparing a quantity of electric power supplied from a power source to a storage battery built in the mobile device with a quantity of electric power consumed by the mobile device (for example, JP 2006-184035A).
A storage battery is also mounted in a vehicle such as an automobile to supply electric power when an engine of a vehicle is started or a quantity of electric power generation of a generator (alternator) is insufficient. It is a recent demand to output electric power from the storage battery to in-vehicle electric devices even when an ignition switch is turned off and the engine is not operated. As one example of such in-vehicle electric devices, a communication device such as a data communication module (DCM) needs continuous power supply to perform its tracking function when the vehicle is stolen.
When the battery runs down due to the continuous power supply to the in-vehicle electric devices with the engine being at rest, the engine occasionally cannot be restarted even if it need be restarted. It is therefore desired to enable detection of the remaining charge of the storage battery while the engine is at rest.
For detecting the remaining battery charge for the mobile devices, a complicated detection circuit configuration is needed since the remaining battery charge is detected by comparison of electric power supplied to the battery with electric power consumed by the electric devices. In case that the remaining battery charge is detected while the engine of the vehicle is at rest, it is desired to simplify the circuit configuration and reduce the power consumption.